1. Field of the Invention
The present invention relates to a stainless steel member and a method for manufacturing the same and, more particularly, to a stainless steel member suitable for a structural member used in highly corrosive environments, such as an edge seal plate to be incorporated together with an electrolyte body into a molten carbonate fuel cell and brought into contact with the peripheral portion of the electrolyte body, and a method for manufacturing the same.
2. Description of the Related Art
Conventionally, various types of fuel cells have been proposed and put into practical use. A molten carbonate fuel cell, among other fuel cells, has been researched and developed extensively because it has a high efficiency and makes the use of coal gas as a fuel gas possible.
The molten carbonate fuel cell has a structure in which a plurality of unit cells, each constituted by a fuel electrode, an air electrode, an electrolyte body, and a collector plate, are stacked with separators arranged between them. Each separator has two edge seal plates so arranged as to clamp the peripheral portion of the electrolyte body containing molten carbonates. The edge seal plates form a wet seal by contacting the peripheral portion of the electrolyte body to thereby shield the electric power generating parts of the unit cells from the external atmosphere. Spring members apply a predetermined surface pressure from both sides to the portions of the edge seal plates clamping the peripheral portion of the electrolyte body, thereby maintaining the wet seal properties of the edge seal plates.
The edge seal plates have been conventionally made of stainless steel with a relatively high corrosion resistance. SUS316, SUS316L, or SUS310S, among other stainless steel materials, is particularly widely used because of its very high corrosion resistance.
The edge seal plates contacting the peripheral portion of the electrolyte body, however, are in direct contact with the molten carbonates which are highly corrosive. Therefore, stainless steel as the material of the edge seal plates corrodes, forming a corrosion product. The corrosion product on any two edge seal plates sandwiching the electrolyte body contact each other. Furthermore, the corrosion products contact the fuel electrode and the air electrode sandwiching the electrolyte body, respectively. Consequently, a short circuit is formed between the fuel electrode and the air electrode, inevitably shortening the lifetime of the cell.
For this reason, it is common practice to manufacture edge seal plates improved in corrosion resistance by coating aluminum on the surface of a base material consisting of stainless steel and performing a heat treatment at a temperature of 900.degree. C. or more to diffuse the aluminum into the base material, thereby forming a corrosion-protective layer. Since, however, cracks reaching the stainless steel as the base material are formed in the corrosion-protective layer, corrosion develops at the portions of the stainless steel where the cracks have reached, adversely affecting the life of a cell.